This application claims the priority of German Application No. 196 35 809.4, filed Sep. 4, 1996, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for improving driving stability of a vehicle during deceleration by detecting wheel speeds of driven and nondriven wheels.
German patent document DE 35 28 389 A1 discloses a device of this generic type which comprises first and second rpm sensors, with the first rpm sensor generating a first rpm signal that corresponds to the rpm of the driving wheel (or the output rpm of the transmission), and the second rpm sensor generating a second rpm signal that corresponds to the rpm of the towed non-driven wheel. The two wheel rpm signals are compared with one another and if the first rpm signal represents an rpm that is higher by at least a predetermined differential value than the second rpm signal, a friction clutch located between the engine and the transmission is disengaged. If the first and second rpm signals differ by less than the predetermined differential value, the friction clutch is re-engaged.
It is possible with the above device in particular to achieve improved driving stability during acceleration in cases when the force transmitted by the driving motor to the drive wheels is greater than the frictional adhesion between the drive wheels and the road. Above a certain amount of slip, a forced decoupling takes place in the drive train which contributes to safe driving behavior.
However, excessive engine braking during deceleration is also problematic, for example if the driven wheels are braked excessively when the accelerator is released. If the drive wheels are braked to such an extent that the adhesive friction between the wheels and the road is lost, lateral guidance can be lost on curves. The development of such situations in vehicles without clutch pedals can be dangerous, since the driver has no way to manually intervene in a crisis situation. In such a vehicle, therefore, in the problem situation described above, understeer or oversteer, or in extreme cases a skid, would unavoidably occur.
The object of the present invention is to provide an improved method and apparatus of the type recited at the outset, which provide improved driving stability of a vehicle, even during deceleration.
This object is achieved by the method and apparatus according to the invention, in which, when the second (driven) wheel rpm is smaller by a certain amount than the first (non-driven) wheel rpm, a friction clutch in the drive train is disengaged so that the flow of power from the engine to the driven wheels is interrupted. As a result of this decoupling of forces, the wheel rpms of the two axles can become equal to one another once more and the clutch can be re-engaged. In this way, it is possible to increase driving stability in critical situations during deceleration, and reliably to prevent understeer or oversteer. This is especially important when the driver has no means of intervention, for example when there is no clutch pedal.
According to one advantageous embodiment, the friction clutch is re-engaged when the driven and nondriven wheels reach essentially the same rpm; in other words when the difference between them is essentially zero. In this case, there is no longer any slip between the drive wheels and the road so that optimum driving stability is ensured.
The difference between the first and second wheel rpm values that determines when the clutch is disengaged preferably depends upon various conditions. First, the limiting value of the engine rpm can be selected. Second, weighting by various vehicle operating conditions can also be provided such as, for example, transverse acceleration and/or the gear that has been engaged. This has the advantage that the slip of the drive wheels during deceleration is evaluated, taking into account the gear that has been engaged or the transverse acceleration, so that these vehicle operating conditions or other vehicle operating conditions can have a critical influence on vehicle stability.
According to one advantageous embodiment of the invention, the disengagement of the friction clutch can take place quickly. On the other hand, when the friction clutch is re-engaged, caution is advisable. For example if the vehicle is traveling over smooth ice, rapid engagement of the friction clutch would produce immediate repeated lock-up of the corresponding drive wheel. For this reason it is advantageous for the engagement of the friction clutch to be regulated. For example, the first and second wheel rpms could be compared continuously with one another during the entire engagement process and a reaction with respect to further engagement could take place if it could be determined how the drive wheel or wheels behave(s) during the engagement process. If for example the rpm of the corresponding drive wheels immediately decreases again during the engagement process, the friction clutch must not be engaged further or must be disengaged once again.
Preferably, all four wheels of a vehicle are monitored, with the first and second wheel rpm values being compared separately for each side of the vehicle in an especially advantageous embodiment of the invention. This ensures a high degree of safety in evaluating vehicle stability during deceleration, especially when the larger of the two differences is used to evaluate the vehicle state.
The apparatus according to the invention requires rpm sensors that pick up the wheel rpm of the corresponding wheels, as well as a comparator for the corresponding wheel rpm signals and a control device that reacts depending on the results of this comparison as described above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.